Valves of numerous specific types are constructed of a hollow valve body having an inlet, an outlet and an internal annular valve seat. The valve seat coacts with the sealing surface of a valve plug to restrict fluid flow through the valve. A valve plug is mounted upon the forward end of a valve stem. The valve stem has a rearward portion being mounted in the valve body for reciprocal movement along a longitudinal axis perpendicular to and coaxial with the valve seat. Commonly the valve stem is in threaded engagement with the valve body and has a hand wheel or knob on its outer rearward end which projects through the valve body.
In operation, therefore, by turning the handwheel, the valve plug is moved forwardly toward the valve seat to restrict fluid flow and is moved rearwardly away from the valve seat to increase fluid flow through the valve. To fully seal the valve the resilient sealing surface of the valve plug is brought into sealing engagement with the valve seat.
It has long been recognized that high velocity extremely turbulent fluid flow results in the vicinity of the valve seat. Turbulent flow of high velocity is pronounced when the longitudinal gap between the valve seat and plug sealing face is relatively small, such as when the valve is initially opened or immediately before complete closing of the valve. Balancing valves used to balance flows within a fluid circuit are especially subject to turbulent flow since the gap between the seat and sealing face is often set relatively narrowly to restrict flow through the valve. Balancing valves therefore, may be subjected to extremely turbulent flows throughout their service lives. Uncontrolled turbulent flow in the vicinity of the valve seat is detrimental for a variety of reasons. Rapid erosion of the valve seat, valve body and plug may result from cavitation caused by the implosion of vapour bubbles resulting from the fluid flow. Upon the collapse of such bubbles, noise is emitted which may be objectionable. If the flow of fluid is not distributed about the periphery of the plug in a substantially equal manner, the flow of fluid may cause unbalanced forces upon the plug and stem which results in lateral mechanical vibrations or chatter. The vibrations cause wearing of the plug stem and stem packing as well as the threads interconnecting the stem and valve body.
In order to evenly distribute fluid flow about the periphery of the valve plug and to reduce the noise, vibrations and cavitation caused by fluid flow in the vicinity of the valve seat, conventional valves have been constructed having grooves, channels or baffles in the valve plug, or in the valve body adjacent the valve seat.
A conventional valve having randomly arranged and shaped serrations in the valve plug is described in the U.S. Pat. No. 3,857,542 to Heymann. The serrations are cut into a skirt of the plug to provide a multiplicity of differently shaped and oriented flow passages which result in random shock structures inhibiting noise and vibration. The consequent dissipation of the high velocity flow and reduction in turbulence reduces noise and mechanical vibration in such valves. Since the valve seat is generally perpendicular to the longitudinal axis of the valve stem and to the direction of fluid flow through the valve, the fluid is forced to change direction as it flows over the valve seat.
As well known to those skilled in the art of fluid dynamics, fluid flow over a sharp edge results in the formation of vapour plumes and unstable vortices. Such vapour plumes contain vapour bubbles caused by low pressure as the fluid flows over the sharp edge. As described above, vapour bubble formation is undesirable especially adjacent valve components since implosion of the vapour bubbles causes cavitation erosion. Unstable vortices are undesirable since vibration of the plug results. To lessen the tendency of vortex and plume formation therefore, the inner edge of the valve seat and outer mating surface of the plug in Heymann are rounded. Conventional valve seats are constructed as replaceable annular discs of uniform thickness countersunk within the adjacent valve body. Rounding of the inner edge of the seat adds to the manufacturing costs of seat and increases the contact area which engages the valve plug sealing face further aggravating the abrasion of the valve seat.
Other conventional valves having various arrangements of grooves to reduce noise and vibration are disclosed in U.S. Pat. No. 2,649,273 to Honegger, U.S. Pat. No. 4,065,095 to Johnson and U.S. Pat. No. 4,763,687 to Arth et al. All such conventional valves suffer from the disadvantage that the valve body or seat design must deviate significantly from the industry standard.
It is desirable therefore, to produce a valve having noise and vibration suppression capabilities which does not deviate from industry standard designs of valve seats and valve bodies in order to allow continued use of existing valve body castings and moulds. It is desirable to produce a valve with a valve seat which is of simple design and is quickly replaceable as in industry standard valves to reduce maintenance costs.
In addition, it is also desirable to enable retrofitting of noise and vibration suppression capabilities within existing valves lacking such capabilities. Retrofitting is most desirable since costs of replacement and removal are avoided.